Presently three main systems of radio transmission are used in radiotelephone communication systems. Each of these systems are intended to make the most efficient use of the allocated frequency spectrum for radio telephone service.
Frequency Division Multiple Access (FDMA) is presently the oldest and most predominant transmission technology used for radiotelephone communication systems. With FDMA the available frequency is divided into many sub-bands. Each individual sub-band serves as an individual radiotelephone communication channel. The transmission and receiving equipment differentiates the different channels by using bandpass filtering techniques.
The transmission technology proposed for recent radiotelephone communication systems uses Time Division Multiple Access (TDMA) for differentiating the various individual channels. In TDMA a period of transmission time is divided into a plurality of time slots. Each communication channel is defined as a particular time slot within that period of transmission time.
When Code Division Multiple Access (CDMA) is used in a radiotelephone transmission system in which all the channels are broadcast at a common broadband frequency, each channel is differentiated by a unique spreading code. The signal is modulated with a spreading code before it is transmitted. The application of the spreading code converts the signal into a broadband signal which is transmitted. Application of this spreading code at the receiver to the received broadband signal demodulates the broadband signal and allows recovery of the information signal of that particular spreading code. This spreading code is typically a binary code comprised of binary bits. The bit pulse intervals are called "chips", which is the technical expression for the pulse periods.
CDMA, in contrast to FDMA and TDMA, advantageously allows each signal to use all of the available time and bandwidth. Since the spreading code uniquely identifies each channel, signals in the other channels appear as noise after the received signal is demodulated by the spreading code.
A widely used CDMA communication system is disclosed in the U.S. Pat. No. 4,901,307 issued Feb. 13, 1990 and entitled "Spread Spectrum Multiple Access Communication System Using Satellite of Terrestrial Repeaters". In the DSSS system each information bit is modulated by a pseudo-random noise sequence (PN sequence) before it is modulated for radio transmission. The modulation with the PN sequence is designated as a spreading process because it produces a signal having a very wide bandwidth compared with the information signal. This same identical wide bandwidth is shared by all the users of a particular communication system.
Such a CDMA system is intrinsically an anti interference system in that any multipath interference having a delay longer than one bit duration of the PN sequence is reduced to a noise level. With the same wide bandwidth available to all users, it permits the use of sophisticated channel coding techniques in order to achieve a very low energy to noise density ratio E.sub.b /N.sub.0 on a per bit basis. This advantageously reduces the interference of the overall system and hence increases the channel capacity of the system. By judicious control of the transmission power, frequency reuse efficiency can be significantly increased as compared with a FDMA system.
As compared with FDMA and TDMA systems in which system capacity is limited by the number of frequency sub-bands and number of time slots, the capacity of a CDMA system is limited by the code generated interference of the non-information signals between users of the system. As the number of users increases the non-information interference increases. This particular interference is designated multiple access interference. This interference is generated in part by application of a despreading code to the non-information portion of the signal received by the receiver. System capacity is determined to a significant extent by this multiple access interference. Hence a direct reduction in the effects of this multiple access interference can greatly increase system channel capacity.
Some existing CDMA systems have found ways to reduce the multiple access interference as a means of increasing channel capacity. These techniques use orthoginal functions as a means of reducing multiple access interference. However these existing techniques are only applicable to CDMA systems in which all the signals can be synchronized. When the signals are not synchronized such as in the reverse link of a cellular radiotelephone system, this method is not effective.